Lecture 5 ( Thermochemistry)

Energy

The capacity to supply heat or do work.

Law of conservation of energy(1st Law of thermodynamics):

during a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form (ΔU= q+w)

What are the 2 main types of energy?

Potential Energy & Kinetic Energy

Potential energy (PE)

the energy an object has because of its relative position, composition, or condition.

Kinetic energy (KE)

the energy that an object possesses because of its motion.

Thermal energy

kinetic energy associated with the random motion of atoms and molecules ( Thermometers are speedometers for atoms)

How does Thermal Energy Flow

Thermal energy always flows from the high T substance to the low T substance

Temperature

= average kinetic energy

Fast moving molecules ➔ “Hot”

Slow moving molecules ➔ “Cold”

calorie (cal)

amount of energy required to raise one gram of water by 1 °C (or 1 kelvin).

Calorie (with a capital C),

large calorie, commonly used in quantifying food energy content, is a kilocalorie.

joule (J)

amount of energy used when a force of 1 newton moves an object 1 meter.

What is the ratio of Joules to calories

1cal=4.184J

Enthalpy (ΔH)

total heat content of the system

Heat (q)

transfer of thermal energy between two bodies at different temperatures.

The system

In chemistry, the system is usually defined as the substances involved in chemical and physical changes.

The surroundings

In chemistry, the surroundings generally include the solvent and container.

exothermic process

A change that releases heat into the surroundings (-q)

endothermic process

A change that absorbs heat from the surroundings (+q)

Closed System

Only Energy can go into and out of a closed system, but matter cannot

Open System

Matter and Energy can Be transferred into and out of the system

Isolated System

neither energy nor matter can enter or leave

Insulated System

Minimized heat transfer, but possible transfer of work and matter.

heat capacity (C)

quantity of heat (q) absorbed or released when a body of matter experiences a temperature change (ΔT) of 1 °C (or 1 kelvin):

specific heat capacity (c)

quantity of heat required to raise the temperature of 1 gram of a substance by 1 °C (or 1 kelvin)

molar heat capacity (c_m)

quantity of heat required to raise the temperature of 1 mole of a substance by 1 °C (or 1 kelvin)

If a substance gains thermal energy,

the final temperature is larger than the initial temperature (Tfinal > Tinitial), then the value of q is positive (q>0).

If a substance loses thermal energy,

the final temperature is smaller than the initial temperature (Tf< Ti), then the value of q is negative (q<0).

calorimeter

a device used to measure the amount of heat involved in a chemical or physical process.

What type of system does a calorimeter form

The inside of a calorimeter forms a closed and insulated system (qtot = 0).

If the reaction is exothermic,

he heat produced by the reaction is absorbed by the solution.

If the reaction is endothermic

the heat required for the reaction to occur is provided by the solution,